Metal oxide catalysts

ABSTRACT

Solid oxide solutions of the formula La1-xSrxCrO3- DIFFERENTIAL  have been found to exhibit surprisingly high specific activity as catalysts for oxidation and hydrogenation reactions. The oxide solutions can be utilized in particulate form or applied as films to the surfaces of refractory substrates. The oxide solution catalysts are expected to be a viable alternative to the noble metal catalysts now used commercially for treatment of combustion chamber exhaust gases to reduce environmental pollutants.

This is a continuation of application Ser. No. 723,848, filed Jul. 1,1991, now abandoned, which is a division of application Ser. No.07/343,054, filed Apr. 25, 1989 now U.S. Pat. No. 5,028,404 issued Jul.2, 1991.

This invention relates to catalyzed oxidation and reduction reactions.More particularly, this invention is directed to improved oxidation andhydrogenation reactions based upon the discovery of exceptionalcatalytic properties of certain solid solutions of lanthanum, strontiumand chromium oxides.

Over the last two decades there has been a significant research anddevelopment effort directed to the advancement of technology forfacilitating air pollution abatement. One area of focus by scientistboth in industry and in educational institutions has been development ofcatalysts for use in catalytic converters for automotive exhaustsystems. Noble metal catalysts, because of their unrivaled catalyticactivity, and in spite of their high cost, have enjoyed wide commercialuse in pollution abatement systems in the automotive industry.Notwithstanding significant research expenditures, there has been littlesuccess toward the development of catalysts or catalyst systems that cancompete with noble metal catalyst with respect to specific activity andresistance to sulfur poisoning. There is a continuing need for thedevelopment of alternative catalysts and catalytic systems forapplications where catalysts of high specific activity are desirable topromote oxidation or hydrogenation reactions.

The use of metal oxides as catalysts for oxidation and reductionreactions are known in the art. However, metal oxide catalysts are verysusceptible to sulfur poisoning. Moreover, they are susceptible tosintering at elevated temperatures. Prior to the discovery underlyingthe present invention, no metal oxide catalyst was recognized to havespecific activity and sulfur poisoning resistance comparable to thatexhibited by noble metal-based catalyst systems.

Therefore, it is one object of this invention to provide lanthanumstrontium chromite compositions adapted for use as catalysts foroxidation and hydrogenation reactions at elevated temperature.

Another object of this invention is to provide a non-sintering metaloxide catalyst which not only has a specific activity comparable tonoble metal catalysts, but also has exhibited good resistance to sulfurpoisoning.

It is another object of this invention to provide an improvement inmetal oxide catalyzed oxidation reactions.

One further object of this invention is to provide an improvement inmetal oxide catalyzed hydrogenation reactions.

It is still another object of this invention to provide a process fortreating exhaust gases from combustion systems by utilizing lanthanumstrontium chromites to catalyze reactions of said gases to reduce levelsof air polluting components.

It has been discovered that solid oxide solutions of the formulaLa_(1-x) Sr_(x) CrO₃₋∂ (where ∂ is a deviation from stoichiometry and xis greater than zero and less than 0.5), have been found to have highspecific activity as catalysts for oxidation reactions. Those samelanthanum strontium chromites have also been found to serve as effectivecatalysts for hydrogenation reactions, for example, hydrogenation ofpropylene at >200° C. Solid solutions of the composition La₀.8 Sr₀.2CrO₃₋∂ and La₀.7 Sr₀.3 CrO₃₋∂ have been found to have a superiorresistance to sintering. Further, they rival the noble metal catalystsin terms of specific activity and immunity to sulfur poisoning. Theabove defined lanthanum strontium chromites are used in accordance withthe improvement of this invention as substitutes for noble metalcatalysts for oxidation and hydrogenation reactions, particularly inapplications requiring high specific catalyst activity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents light-off temperature curves of carbon monoxideoxidation for four catalyst compositions.

FIG. 2 presents light-off temperature curves of propylene hydrogenationfor three catalyst compositions.

DETAILED DESCRIPTION OF THE INVENTION

There is provided an improved method for conducting oxidation andhydrogenation reactions in the presence of a metal oxide catalyst. Solidoxide solutions of the formula La_(1-x) Sr_(x) CrO₃₋∂, wherein x isgreater than zero and less than about 0.5, exhibit specific catalystactivities comparable to those exhibited by noble metal catalysts. Theterms "specific activity" or "specific catalyst activity" as used inthis description refers to moles reacted per unit time per exposedsurface area. Preferred solid oxide solutions in accordance with thepresent invention have a stoichiometry represented by the above formulawherein x is about 0.1 to about 0.4.

In accordance with art-recognized practices, the term "∂" in the oxygensubscript of the metal oxide formula is an index of deviation from theindicated oxygen stoichiometry so that the whole number 3 is the wholenumber closest to the numerical value of the oxygen subscript (3-∂). Inother words, the absolute value of ∂ (|∂|) is less than 0.5.

The lanthanum strontium chromite catalysts useful in accordance withthis invention can be prepared by any of a wide variety ofart-recognized techniques for forming solid metal oxide solutions. Thus,for example, the component metal oxides can be combined in the desiredstoichiometric proportions, pelletized and sintered at temperaturesranging between about 900° C. to about 1100° C., optionally followed bycycles of grinding, pelletizing and sintering to ensure completereaction and solid solution homogeneity.

The present lanthanum strontium chromite catalysts can also be preparedas a coating or film on refractory substrates, preferably refractorysubstrates exhibiting a surface area of greater than 0.5 m² /gram.Exemplary of refractory substrates suitable as carriers for films orcoatings of the lanthanum strontium chromite useful in accordance withthe present invention include quartz, porcelain, silicon carbide,crystallized glasses, alumina and the like. Films of the present oxidesolutions can be applied to the surface of such substrates utilizingart-recognized plasma deposition technology or by so-called metalorganic decomposition techniques, such as those described in U.S. Pat.No. 3,658,568, issued Apr. 25, 1972 and U.S. Pat. No. 4,485,094, issuedNov. 27, 1984, the disclosures of which patents are expresslyincorporated herein by reference. Generally lanthanum strontiumchromites for use in accordance with this invention can be applied asthin films to refractory materials by first applying a solution of amixture of soluble metal salts or metal complexes to the surface of saidsubstrates followed by oxide-forming pyrolysis. The stoichiometry of theresulting solid oxide solution film corresponds to the stoichiometry ofthe metal organic components of the precursor solutions. It isanticipated that the refractory substrates for deposition of lanthanumstrontium chromites for use in accordance with this invention can havesimilar structure/geometry to those substrates used heretofore forsupport of noble metal catalysts intended for use in similarapplications.

In a preferred embodiment of this invention, lanthanum strontiumchromites of the above defined stoichiometry can be used as an effectivecatalyst for treatment of automotive exhaust and allied pollutionabatement processes. Thus the solid metal oxide solutions can beutilized as sintered pellets or as films or coatings on conventionalrefractory substrates in the same manner as noble metal catalysts arepresented in the commercial catalytic converters now enjoying wide usein the automotive industry. The present lanthanum strontium chromitescan be used as a cost effective alternative to art-recognized noblemetal reforming catalysts and other hydrotreating catalytic agents.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Solid solutions of the general formula La_(2-x) Sr_(x) CuO₄₋∂ andLa_(1-x) Sr_(x) CrO₃₋∂ were prepared by conventional ceramic techniques.The constituent metal oxides were quantitatively mixed in the desiredstoichiometric ratio and pelletized. Each pellet was sintered in analumina crucible in air for 12 hours at the appropriate temperature. Thesintering temperature for the cuprates was 1050° C. to 1100° C., whilethe sintering temperature for the chromites was 900°-920° C. Cycles ofgrinding, pelletizing and heating were repeated three times on eachsample to ensure complete reaction and solution homogeneity. X-raydiffraction, SEM and EDX showed that each sample was a homogeneoussingle phase. The component oxides used to prepare the solid solutionswere high purity (99.99%) from Puratronic Specialty Products from AESAR(Johnson Mathey). In each instance the final pellets were ground and asieve fraction between 10 μm and 35 μm was used for the catalyticstudies. Surface area of the solid solutions was measured by the BET N₂adsorption technique.

For the catalytic studies, 0.5% platinum on γ- alumina, supplied byEnglehart Industries, Inc., was utilized as a control catalyst. Platinumsite counting on the catalyst was accomplished utilizing carbon monoxidechemisorption as well as hydrogen-oxygen titration.

Temperature programmed catalytic studies were carried out in amulti-functional in situ catalyst characterization unit at theUniversity of Notre Dame. The tests were made utilizing a fixed feedconcentration of gas mixtures (1% CO in oxygen) through a fixed amountof powdered catalyst (0.25 g) to maintain constant contact time. Theconversion of carbon monoxide was determined as a function oftemperature while the catalyst was programmably heated at a rate of 2°C. per minute. The effluent from the gradientless recycling reactor wassampled using a zero volume sampling valve at different time intervals,and was analyzed by means of a gas liquid chromatograph interfaced withan automatic integrator.

                  TABLE 1                                                         ______________________________________                                        Activity of the oxidation catalysts for CO oxidation*                                                   Activity (Temperature                                                         for 50% CO                                          Catalyst**    La/(La + Sr)                                                                              conversion) (°C.)                            ______________________________________                                        CuSrO.sub.2   0           227                                                 LaSrCuO.sub.4-∂ ***                                                            0.5         212                                                 La.sub.1.6 Sr.sub.0.4 CuO.sub.4-∂                                              0.8         192                                                 La.sub.1.8 Sr.sub.0.2 CuO.sub.4-∂                                              0.9         202                                                 La.sub.1.9 Sr.sub.0.1 CuO.sub.4-∂                                              0.95        201                                                 La.sub.2 CuO.sub.4-∂                                                           1.0         240                                                 CuO           --          146                                                 La.sub.2 O.sub.3                                                                            --          192                                                 SrO           --          244                                                 ND1 (La.sub.0.8 Sr.sub.0.2 CrO.sub.3-∂)                                        --          122                                                 ND2 (La.sub.0.7 Sr.sub.0.3 CrO.sub.3-∂)                                        --          139                                                 0.5% Pt/Alumina                                                                             --           98                                                 ______________________________________                                         *Flow rate 100 ml/min of a 1% CO in oxygen at normal temperature and          pressure                                                                      **Weight of the catalyst = 0.25 g.                                            ***.sub.∂  is an index of deviation from stoichiometry.     

The measured activity of the various metal oxide catalysts are reportedin Table 1 as the temperature at which 50% conversion of carbon monoxidewas reached for a constant amount of catalyst (0.25 g). Generally, solidsolutions of La_(2-x) Sr_(x) CuO₄₋∂ were determined to be less reactivethan those of La_(1-x) Sr_(x) CrO₃₋∂. Further, the solid solutionexhibited a significant difference in catalytic activity from the testedcomponent metal oxides and that generally the lanthanum strontiumchromites were found to be more active than the corresponding cupratesfor carbon monoxide oxidation. Indeed, the activity of the solidsolutions of the general formula La_(1-x) Sr_(x) CrO₃₋∂ was found to becomparable to that of platinum/alumina catalysts. Two of the lanthanumstrontium chromites, ND1 (x=0.2) and ND2 (x=0.3) were synthesized andthe catalytic kinetics of carbon monoxide oxidation on thesecompositions were evaluated. The results of that study are summarized inFIG. 1 which illustrates the light-off temperature curves of carbonmonoxide oxidation for ND1(a); ND2(b); La₁.6 Sr₀.4 CuO₄₋∂ (c), andplatinum/alumina catalyst(d) . The reported values were obtained for a1% carbon monoxide concentration. The activity period of the lanthanumstrontium chromites ND1 and ND2 is shown in FIG. 1 to be very similar tothat of the platinum/alumina catalyst. Further, the kinetic studies ofcarbon monoxide oxidation on these solid solutions appear to follow thesame Langmuir-Hinshelwood model as that of the platinum/aluminacatalyst. It is noted that the upper limit of the light-off temperaturecurves for solid solution ND1 evidences a carbon monoxide conversionsuperior to that of the platinum catalyst.

Additional data were obtained for comparison of the specific activity ofND1 and ND2 relative to that of the platinum/alumina catalyst. Theresults of those additional studies are summarized in Table 2. In thecase of the metal oxide catalysts, the BET area was used to calculatethe specific activity, as it was not possible to find a suitable probeto count the active sites specifically.

It is noted that the BET area can be controlled by time and temperatureof catalyst preparation. Higher BET areas can be realized by using lowercatalyst preparation temperatures with longer heating periods. ElevatedBET areas should reduce temperatures required for effective catalystactivity for both oxidation and hydrogenation reactions.

One problem suffered by many catalysts is their susceptibility to sulfurpoisoning in oxidizing atmospheres. Testing performed on metal oxidesolution catalysts ND1 has shown that its exposure to hydrogen sulfideat room temperature for 12 hours resulted in no loss of carbon monoxideoxidation activity.

The activity of the lanthanum strontium chromite ND1 (x=0.2) was alsocompared with platinum/alumina catalysts for propylene hydrogenation.FIG. 2 illustrates the results of that evaluation for a system in whichthe feed contained 3.8% propylene. The light-off temperature curve forND1 [(a) in FIG. 2] indicates that that metal oxide solid solution has akinetic catalytic activity very similar to both fresh 0.5% Pt/Alumina(b) and a Pt/Alumina catalyst sintered at 600° C. (c).

                  TABLE 2                                                         ______________________________________                                        Specific activity of the oxidation catalysts                                                Wt. of                 Specific.sup.+                                         the cat-                                                                              BET            activity                                               alyst   Area    Exposed                                                                              (moles of                                Catalyst      (g)     (m.sup.2 /g)                                                                          area (m.sup.2)                                                                       CO/m.sup.2 h)                            ______________________________________                                        Pt/Alumina    0.12    249     0.27@  0.04                                                                   0.47*  0.02                                     ND1 (La.sub.0.8 Sr.sub.0.2 CrO.sub.3-∂)                                        0.25    0.33    0.08** 0.02                                     ND2 (La.sub.0.7 Sr.sub.0.3 CrO.sub.3-∂)                                        0.25    0.35    0.09** 0.02                                     ______________________________________                                         .sup.+ Specific rate is moles of CO reacted per unit contact time per         exposed area. As the difference in the temperature range for 50%              conversion is very close, the conversion was compared at different            temperatures, the error involved is very slight compared to the magnitude     @Specific surface area of Pt measured by CO chemisorption technique           *Specific surface area of Pt measured by hydrogenoxygen titration             technique                                                                     **BET Area × g                                                     

We claim:
 1. A catalyst composition comprising a refractory substrateand on the surface of said substrate a metal oxide solution of theformula La_(1-x) Sr_(x) CrO₃₋∂ wherein x is greater than zero and lessthan about 0.5 and ∂ is an index of deviation from the indicated oxygenstoichiometry so that the whole number 3 is the whole number closest tothe numerical value of the subscript 3-∂.
 2. The catalyst of claim 1wherein x is about 0.1 to about 0.4.
 3. The catalyst of claim 1 whereinx is about 0.2.
 4. The catalyst of claim 1 wherein x is about 0.3. 5.The catalyst of claim 1 wherein the refractory substrate has a surfacearea greater than 0.5 square meters per gram.
 6. The catalyst of claim 1wherein the refractory material is a porous ceramic.